Semiconductor devices and liquid crystal display devices are fabricated by a so-called photolithography technique, wherein a pattern formed on a mask is transferred onto a photosensitive substrate.
An exposure apparatus used in this photolithographic process has a mask stage that supports a mask, as well as a substrate stage that supports a substrate, and transfers the pattern of the mask onto the substrate through a projection optical system while successively moving the mask stage and the substrate stage.
There has been demand in recent years for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. The shorter the exposure wavelength used and the larger the numerical aperture of the projection optical system, the higher the resolution of the projection optical system. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has increased. Furthermore, the mainstream exposure wavelength currently is the 248 nm KrF of excimer laser, but an even shorter wavelength 193 nm of ArF excimer laser is also being commercialized. In addition, as with resolution, the depth of focus (DOF) is important when performing an exposure. The following equations express the resolution R and the depth of focus δ, respectively.R=k1λ/NA  (1)δ=±k2·λ/NA2  (2)
Therein, λ, is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are the process coefficients. Equations (1) and (2) teach that if the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ decreases.
At this time, if the depth of focus δ becomes excessively small, then it will become difficult to align the front surface of the substrate with the image plane of the projection optical system, and there will be a risk of insufficient margin of focus during the exposure operation.
Accordingly, a liquid immersion method has been proposed, as disclosed in, for example, Patent Document 1 below, as a method to substantially shorten the exposure wavelength and increase the depth of focus. This liquid immersion method forms a liquid immersion area by filling a gap between the lower surface of the projection optical system and the front surface of the substrate with a liquid, such as water or an organic solvent, thus taking advantage of the fact that the wavelength of the exposure light in a liquid is 1/n that of in air (where n is the refractive index of the liquid, normally about 1.2 to 1.6), and thereby improving the resolution as well as increasing the depth of focus by approximately n times.
Patent Document 1: PCT International Publication WO99/49504
Incidentally, it is important in an immersion exposure apparatus to form a liquid immersion area in a desired state. For example, if the liquid immersion area on the substrate is not formed in a desired state, then problems will arise, such as the degradation of the pattern image or the failure of the exposure light to reach the substrate, which will result in a degradation of the exposure accuracy. In addition, it is also conceivable that a measurement process will be performed through the liquid using, for example, a measurement member and a measurement sensor, which are provided on the substrate stage; however, even in that case, the measurement accuracy will degrade if the liquid immersion area on the substrate stage is not formed in a desired state.
In addition, if the liquid in the immersion area flows out of the immersion area, or if the liquid used during exposure leaks and then infiltrates and adheres to a position outside of the desired position, then there is a possibility that that liquid will cause problems, such as a failure of apparatuses and members, electrical leakage, and rusting, thereby causing the degradation of exposure accuracy, measurement accuracy, and the like. If, unfortunately, a substrate is loaded onto a substrate holder in a state wherein, for example, liquid is adhering to that substrate holder that holds a substrate, then that liquid will function as a lubricating film and cause the mispositioning of the substrate with respect to the substrate holder, thereby degrading exposure accuracy, measurement accuracy, and the like.
In addition, to satisfactorily form the liquid immersion area and to satisfactorily recover the liquid, it is preferable to maintain the affinity between the liquid and the substrate as well as the upper surface of the substrate stage in an optimal state. If the liquid is not completely recovered and unfortunately remains, then that remaining liquid vaporizes and, for example, causes: thermal deformation of the substrate, the substrate stage, and the like; fluctuations in the environment (temperature, humidity) wherein the substrate is disposed; and fluctuations in the optical paths of the various measurement beams that measure the positional information of the substrate, and the like, thereby degrading exposure accuracy, measurement accuracy, and the like. In addition, after the remaining liquid vaporizes, a water residue (a so-called watermark) is unfortunately formed, which creates an error factor in the various measurements, and there is also a possibility that the substrate and the like will be contaminated by the generation of foreign matter.